Code inspection: Declaration nullability inferred (parameter is inferred to be not null)

This inspection reports a parameter that control-flow analysis infers is never expected to be null. Typical cases are parameters that are immediately dereferenced, validated with ArgumentNullException, or consistently used as non-null throughout the method body. Adding [NotNull] makes that contract explicit for callers and for static analysis.

Example

In this example, the text parameter is validated with ArgumentNullException. The quick-fix suggests adding the [NotNull] attribute.

                using System;

                public class Parser
                {
                    public void Parse(string text)
                    {
                        if (text == null)
                            throw new ArgumentNullException(nameof(text));
                        Console.WriteLine(text.Length);
                    }
                }
            

                using System;
                using JetBrains.Annotations;

                public class Parser
                {
                    public void Parse([NotNull] string text)
                    {
                        if (text == null)
                            throw new ArgumentNullException(nameof(text));
                        Console.WriteLine(text.Length);
                    }
                }
            

Quick-fix

Annotate the parameter with the inferred nullability attribute, usually [NotNull]. For container-like parameters, the quick-fix can use [ItemNotNull] instead.

28 March 2026